If it were possible to have perfect combustion of a hydrocarbon fuel in an automotive internal combustion engine, the only products of such combustion would be carbon dioxide and water. These products have not been found to be harmful. Unfortunately, such perfect combustion is seldom realized and the exhaust gases from a typical internal combustion engine contain various quantities of unburned hydrocarbons, carbon monoxide and oxides of nitrogen. Because of the indeterminate and transitory nature of the nitrogen oxides, these are commonly referred to as NOX. Formation of oxides of nitrogen is enhanced whenever temperatures within the combustion chambers are high. Reduction of the temperature of combustion can be achieved by injecting an inert gas into the air-fuel mixture which enters the individual cylinders of the engine. The introduction of such inert gases results in a reduction of the peak temperatures achieved during the burning process and therefore a reduction in the quantity of NOX produced.
As pointed out above, an inert gas is useful in overcoming the problems associated with excessive production of oxides of nitrogen. Since it is undesirable to use a separate source of inert gases, it is fortunate that the products of combustion in the exhaust system of the vehicle contains the necessary components to make these exhaust gases qualify as an inert gas. Accordingly, the industry has made use of the exhaust gases to reduce NOX emissions. A variety of systems have been used to introduce exhaust gases into the intake manifold of the engine and these range from a simple opening between the exhaust manifold and the intake manifold with no control whatsoever, to somewhat complex control mechanisms that derive a control signal from the carburetor and utilize this signal to, in some manner, control a diaphragm-actuated valve to control the admission of exhaust gases in a regulated manner.
It has been found that it is undesirable to have exhaust gas recirculation into the intake manifold of the engine during certain modes of operation. For example, engine idle may be adversely effected with exhaust gas recirculation and, therefore, some vehicles have been equipped in such a manner that no exhaust gas is injected into the intake during engine idle conditions. Also, the injection of exhaust gases during peak power requirements may reduce the power available and, again, this has been found to be undesirable. Accordingly, some of the systems in present day use will shut off exhaust gases during engine idling and/or during wide open throttle high power conditions. To do this, some exhaust gas recirculation systems have installed a port in the body of the carburetor just above the throttle valve so that when the throttle is at the idle position, no vacuum signal is available to operate the diaphragm valve. Upon opening the throttle slightly, the manifold vacuum is made available by uncovering the port and this, in turn, opens the recirculation valve. Another system in use has been to provide a port in the vicinity of the venturi throat of the carburetor. This signal is weak at low flow rates and becomes stronger as air flow increases. The first system just described does not control the shut-off as well at high flows as may be desirable and the second system operates best at high air flow volumes and, therefore, does not shut-off at high air flow.